1. Field of the Invention
This invention relates generally to the design of an oxygen sensor and more particularly to a zirconia cell oxygen sensor especially designed for use in monitoring respiratory gases and providing an indication of the concentration of oxygen in the respiratory gas on a breath-by-breath basis.
II. Discussion of the Prior Art
In the Anderson et al U.S. Pat. No. 4,463,764, which is assigned to the assignee of the present invention, there is described a cardiopulmonary performance analyzer utilizing distributed processors for controlling the sensing, measurement, computation and display of a wide variety of metabolic parameters on a breath-by-breath basis. One of the components used in that system is an oxygen analyzer. It incorporates a slip cast zirconia cell with hand-painted electrodes. This method of manufacture is very labor-intensive and, therefore, expensive. In addition, the dimensional stability of the cell at the operating temperatures involved is somewhat poor, making it difficult to keep the sample volumes either low or uniform.
It is well known in the art that if a difference in the partial pressure of oxygen exists across a heated ZrO.sub.2 ceramic stabilized by Y.sub.2 O.sub.3, a voltage proportional to the log of the ratio of the partial pressures will be produced in accordance with the Nenst equation. A platinum or other suitable conductive coating on each side of the element provides a means for reading the voltage. Cells of this type are widely used in modern automobiles employing pollution control systems. By placing a zirconia cell in the exhaust manifold of an internal combustion engine, the cell is heated by the hot exhaust gases and is capable of providing a voltage signal indicative of the amount of oxygen present in the exhaust stream. Given the millions of automobiles produced each year and the need to periodically replace the oxygen sensor in older automobiles, zirconia cells for automotive use are produced in vast quantities and at relatively low prices. Thus, a significant cost reduction could be realized if commercially-available automotive zirconia cell oxygen sensors could be made to function reliably in a respiratory gas analyzer where the response time of the cell would have to accommodate real-time, breath-by-breath changes in oxygen concentration.
It is accordingly a principal object of the present invention to provide an improved, low-cost oxygen analyzer for use in respiratory gas analysis systems.
Another object of the invention is to provide a zirconia cell-type oxygen analyzer providing a rapid response to a step change in oxygen concentration of a sample gas at a low volume flow rate.
Yet another object of the present invention is to provide a means whereby a conventional zirconia cell designed for automotive applications can be adapted to a biomedical application involving extremely low sample gas flow rates while providing the requisite fast response times to changes in oxygen concentration.